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The Rotationally Modulated Polarization of Ξ Boo A

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The Rotationally Modulated Polarization of Ξ Boo A View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by University of Southern Queensland ePrints MNRAS 483, 1574–1581 (2019) doi:10.1093/mnras/sty3180 Advance Access publication 2018 November 29 The rotationally modulated polarization of ξ Boo A 1,2‹ 3 3 1,2 Daniel V. Cotton, Dag Evensberget, Stephen C. Marsden, Jeremy Bailey , Downloaded from https://academic.oup.com/mnras/article-abstract/483/2/1574/5218518 by University of Southern Queensland user on 14 November 2019 Jinglin Zhao,1 Lucyna Kedziora-Chudczer ,1,2 Bradley D. Carter,3 Kimberly Bott ,4,5 Aline A. Vidotto ,6 Pascal Petit,7,8 Julien Morin9 and Sandra V. Jeffers10 1School of Physics, UNSW Sydney, NSW 2052, Australia 2Australian Centre for Astrobiology, UNSW Sydney, NSW 2052, Australia 3University of Southern Queensland, Centre for Astrophysics, Springfield, Qld. 4300/Toowoomba, Qld. 4350, Australia 4University of Washington Astronomy Department, Box 351580, UW Seattle, WA 98195, USA 5NExSS Virtual Planetary Laboratory, Box 351580, UW Seattle, WA 98195, USA 6School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland 7Universite´ de Toulouse, UPS-OMP, IRAP, Toulouse F-31400, France 8CNRS, Institut de Recherche en Astrophysique et Planetologie, 14, avenue Edouard Belin, F-31400 Toulouse, France 9LUPM-UMR 5299, CNRS & Universite´ Montpellier, place Eugene` Bataillon, F-34095 Montpellier Cedex 05, France 10Institute for Astrophysics, University of Goettingen, Friedrich Hund Platz 1, D-37077 Goettingen, Germany Accepted 2018 November 16. Received 2018 November 15; in original form 2018 September 25 ABSTRACT We have observed the active star ξ Boo A (HD 131156A) with high precision broadband linear polarimetry contemporaneously with circular spectropolarimetry. We find both signals are modulated by the 6.43 d rotation period of ξ Boo A. The signals from the two techniques are 0.25 out of phase, consistent with the broadband linear polarization resulting from differential saturation of spectral lines in the global transverse magnetic field. The mean magnitude of the linear polarization signal is ∼4 ppm G–1 but its structure is complex and the amplitude of the variations suppressed relative to the longitudinal magnetic field. The result has important implications for current attempts to detect polarized light from hot Jupiters orbiting active stars in the combined light of the star and planet. In such work stellar activity will manifest as noise, both on the time-scale of stellar rotation, and on longer time-scales – where changes in activity level will manifest as a baseline shift between observing runs. Key words: polarization – stars: activity – stars: individual HD 131156A – stars: magnetic field. in three, where the outside lines are polarized in one orientation 1 INTRODUCTION and the centre line – having double the intensity – is polarized The primary mode of characterizing the magnetic field in a star in the other (Stenflo 2013). When the magnetic field is weak, the is through circular polarimetry. In highly magnetic stars linear po- lines are not completely split, but instead the two components are larization may be used to complement measurements of circular to be found predominantly in the line wings and line core, re- polarization, and constrain magnetic field geometry (Wade et al. spectively. Spectropolarimetry – where the line profiles are fit to 2000 and references therein). However, modern stellar polarimeters determine polarization and hence magnetic field strength – can be are much more sensitive to circular polarization than to the inher- used to measure both types of polarization, but the line profiles of ently weaker signal from linear polarization. Only in the last decade circular polarization are much more easily detected (Wade et al. has linear polarization been definitively detected in (bright) weakly 2000). magnetic stars (Kochukhov & Wade 2010;Rosen,´ Kochukhov & In our recent paper we measured significant broadband linear Wade 2015). The difference arises as a result of the polarimetric polarization in a number of active late-type dwarf stars – mostly mechanism. In a magnetic field circular polarization is produced BY Dra variables and stars with emission line spectral types (Cot- by the longitudinal Zeeman effect splitting spectral lines into two ton et al. 2017b). In stars with very strong magnetic fields a net oppositely polarized (left and right handed) lines. Linear polariza- linear polarization will be measured in a line when the line core is tion is produced by the transverse Zeeman effect splitting lines saturated – called magnetic intensification (Babcock 1949). Sim- ilarly, ‘differential saturation’ describes the situation where many lines overlap and merge with each other (line blanketing) to pro- E-mail: [email protected] duce a net broadband linear polarization (Bagnulo et al. 1995). The C 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Polarization of ξ Boo A 1575 broadband linear polarization magnitude measured in the active Table 1. TP determination from low polarization standard observations. dwarfs was correlated with the maximum global longitudinal mag- Exposure times are 320 s for Sirius and 640 s otherwise. netic field (|B|max) from spectropolarimetric (circular polarimetry) measurements. Consequently it is presumed that the broadband lin- Star UTC q (ppm) u (ppm) ear polarization measured in these active dwarfs is produced through β Hyi 2017-06-22 19:21:20 − 25.9 ± 3.4 − 0.9 ± 3.3 differential saturation that is also induced by the global magnetic 2017-06-29 19:38:38 − 22.1 ± 5.0 1.1 ± 4.7 Downloaded from https://academic.oup.com/mnras/article-abstract/483/2/1574/5218518 by University of Southern Queensland user on 14 November 2019 field. If so, the field geometry will be important, a uniform dipolar 2017-06-30 19:50:35 − 15.8 ± 3.9 11.7 ± 3.8 field aligned with the stellar rotation axis might produce a constant 2017-08-11 16:26:58 − 21.5 ± 4.6 21.4 ± 4.4 polarization, more complicated structures will result in a time vary- 2017-08-11 17:48:42 − 19.0 ± 3.9 − 3.7 ± 4.0 ing signal. However, linear polarization may also be generated in 2017-08-17 19:24:21 − 31.5 ± 4.7 2.4 ± 5.2 active stars through other mechanisms with more complicated phase Sirius 2017-08-10 19:49:29 − 18.6 ± 3.6 6.2 ± 3.9 − ± − ± behaviour. Strong localized fields might be produced in starspots 2017-08-15 19:23:35 25.5 8.3 15.7 7.9 2017-08-16 19:27:41 − 19.5 ± 2.7 20.6 ± 3.6 (Huovelin & Saar 1991; Saar & Huovelin 1993). Or starspots might 2017-08-18 19:07:11 − 10.1 ± 12.2 12.8 ± 13.9 produce polarization by breaking symmetry, not in the spectral 2017-08-19 19:33:56 3.0 ± 2.3 − 9.0 ± 2.5 lines, but on the disc of the star instead (Yakobchuk & Berdyug- β Leo 2017-06-22 08:17:33 − 3.1 ± 2.4 − 6.0 ± 2.4 ina 2018). In red super/giant stars, stellar hotspots have also been 2017-06-26 08:17:37 − 8.0 ± 2.4 − 11.9 ± 2.2 found to produce linear polarization (Schwarz 1986;Auriere` et al. 2017-07-05 08:15:41 − 10.1 ± 3.4 − 7.8 ± 2.9 2016). β Vir 2017-06-23 08:14:32 − 2.7 ± 5.1 − 7.6 ± 4.9 Determining the polarimetric mechanism of the linear polariza- 2017-06-24 08:20:36 − 8.8 ± 4.6 − 5.7 ± 4.7 tion in active dwarfs is important, not just for the information com- Adopted TP − 15.0 ± 0.3 0.5 ± 0.3 plementary to spectropolarimetry (e.g. Wade et al. 1996;Rosen´ et al. 2015), but also because it is a potential source of noise in studying other polarimetric phenomena. In particular, a number of groups have been searching for the polarized light that is scattered 2 OBSERVATIONS from the atmosphere of a close hot Jupiter planet, in the combined light of the star and the planet (Lucas et al. 2009; Berdyugina et al. 2.1 Linear polarimetry 2011; Wiktorowicz et al. 2015;Bottetal.2018). If identified, such a signal can reveal details of the planet’s atmosphere: its albedo and Broadband linear polarization measurements were made with the cloud properties. However, stellar activity is likely to significantly HIgh Precision Polarimetric Instrument (HIPPI; Bailey et al. 2015) complicate such searches as the expected signal due to an orbit- on the 3.9-m Anglo-Australian Telescope, at Siding Spring Observa- ing, unresolved exoplanet is smaller than that seen in active dwarfs tory in Australia. The instrument was mounted at the F/8 Cassegrain (Seager, Whitney & Sasselov 2000; Bailey, Kedziora-Chudczer & focus, giving an aperture size of 6.7 arcsec – just small enough to Bott 2018), and many of the best candidate systems for detecting isolate ξ Boo A from ξ Boo B at the time of our observations a planetary signal (those with very short period planets orbiting (it is difficult to measure the seeing with HIPPI on the telescope bright stars) have late-type dwarf star hosts that are active or poten- accurately, but the seeing was decent – generally around 2 arcsec tially active. If activity effects are to be avoided, or removed, it is or better – for our observations). HIPPI has a night-to-night preci- important they be understood. sion of 4.3 ppm on bright stars, which it achieves using a (Boulder As the most polarized star identified in Cotton et al. (2017b) ξ Non-linear Systems) ferro-electric liquid crystal modulator operat- Boo A (HD 131156A) is the most obvious candidate to look for and ing at 500 Hz, and two additional slower stages of chopping (Bailey characterize any variability.
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